Insurance Contracts - Summary of Information Related to Policies Currently Included in Ambac's Loss Reserves or Subrogation Recoverable (Details) $ in Millions	3 Months Ended	12 Months Ended
	Mar. 31, 2024 USD ($) Policies	Dec. 31, 2023 USD ($) Policies	Mar. 31, 2023 USD ($)
Schedule of Insured Financial Obligations with Credit Deterioration [Line Items]
Ceded Loss And Loss Expenses Paid Not Yet Recovered			$ 3
Number of policies | Policies	147	141
Remaining weighted-average contract period (in years)	11 years	12 years
Gross insured contractual payments outstanding:
Principal	$ 3,801	$ 3,838
Interest	1,395	1,421
Total	5,196	5,259
Gross undiscounted claim liability	1,135	1,202
Discount, gross claim liability	445	426
Gross claim liability before all subrogation and before reinsurance	(691)	777
Less:
Gross other subrogation	(259)	(263)
Discount, other subrogation	72	69
Discounted other subrogation, before reinsurance	(187)	(194)
Gross claim liability, net of all subrogation and discounts, before reinsurance	(504)	583
Less: Unearned premium reserves	(24)	(28)
Plus: Loss adjustment expenses reserves	4	4
Claim liability reported on Balance Sheet, before reinsurance	484	559
Reinsurance recoverable reported on Balance Sheet	26	30
Financial Guarantee
Schedule of Insured Financial Obligations with Credit Deterioration [Line Items]
Ceded Loss And Loss Expenses Paid Not Yet Recovered	16
Financial Guarantee
Schedule of Insured Financial Obligations with Credit Deterioration [Line Items]
Loss Reserves Ceded To Reinsurers	26	30
Ceded Loss And Loss Expenses Paid Not Yet Recovered	$ 1	$ (1)
I/SL [Member]
Schedule of Insured Financial Obligations with Credit Deterioration [Line Items]
Number of policies | Policies	22	18
Remaining weighted-average contract period (in years)	7 years	9 years
Gross insured contractual payments outstanding:
Principal	$ 1,086	$ 429
Interest	104	75
Total	1,190	505
Gross undiscounted claim liability	11	1
Discount, gross claim liability	1	0
Gross claim liability before all subrogation and before reinsurance	(10)	1
Less:
Gross other subrogation	(12)	(13)
Discount, other subrogation	2	2
Discounted other subrogation, before reinsurance	(10)	(11)
Gross claim liability, net of all subrogation and discounts, before reinsurance	0	10
Less: Unearned premium reserves	(7)	0
Plus: Loss adjustment expenses reserves	1	0
Claim liability reported on Balance Sheet, before reinsurance	(6)	(10)
Reinsurance recoverable reported on Balance Sheet	$ 1	$ 1
IA [Member]
Schedule of Insured Financial Obligations with Credit Deterioration [Line Items]
Number of policies | Policies	7	8
Remaining weighted-average contract period (in years)	14 years	9 years
Gross insured contractual payments outstanding:
Principal	$ 501	$ 1,084
Interest	312	328
Total	813	1,412
Gross undiscounted claim liability	7	19
Discount, gross claim liability	1	2
Gross claim liability before all subrogation and before reinsurance	(6)	17
Less:
Gross other subrogation	(2)	(2)
Discount, other subrogation	0	0
Discounted other subrogation, before reinsurance	(2)	(2)
Gross claim liability, net of all subrogation and discounts, before reinsurance	(4)	(15)
Less: Unearned premium reserves	(5)	(12)
Plus: Loss adjustment expenses reserves	2	3
Claim liability reported on Balance Sheet, before reinsurance	1	6
Reinsurance recoverable reported on Balance Sheet	$ 0	$ 0
II [Member]
Schedule of Insured Financial Obligations with Credit Deterioration [Line Items]
Number of policies | Policies	11	9
Remaining weighted-average contract period (in years)	12 years	13 years
Gross insured contractual payments outstanding:
Principal	$ 442	$ 430
Interest	264	262
Total	707	692
Gross undiscounted claim liability	37	41
Discount, gross claim liability	6	7
Gross claim liability before all subrogation and before reinsurance	(31)	34
Less:
Gross other subrogation	(1)	0
Discount, other subrogation	0	0
Discounted other subrogation, before reinsurance	0	0
Gross claim liability, net of all subrogation and discounts, before reinsurance	(31)	34
Less: Unearned premium reserves	(4)	(4)
Plus: Loss adjustment expenses reserves	0	0
Claim liability reported on Balance Sheet, before reinsurance	26	30
Reinsurance recoverable reported on Balance Sheet	$ 7	$ 8
III [Member]
Schedule of Insured Financial Obligations with Credit Deterioration [Line Items]
Number of policies | Policies	11	13
Remaining weighted-average contract period (in years)	11 years	13 years
Gross insured contractual payments outstanding:
Principal	$ 276	$ 394
Interest	103	139
Total	379	534
Gross undiscounted claim liability	230	324
Discount, gross claim liability	76	86
Gross claim liability before all subrogation and before reinsurance	(154)	239
Less:
Gross other subrogation	(26)	(27)
Discount, other subrogation	4	4
Discounted other subrogation, before reinsurance	(22)	(23)
Gross claim liability, net of all subrogation and discounts, before reinsurance	(133)	215
Less: Unearned premium reserves	2	0
Plus: Loss adjustment expenses reserves	0	0
Claim liability reported on Balance Sheet, before reinsurance	135	215
Reinsurance recoverable reported on Balance Sheet	$ 14	$ 18
IV [Member]
Schedule of Insured Financial Obligations with Credit Deterioration [Line Items]
Number of policies | Policies	91	88
Remaining weighted-average contract period (in years)	12 years	12 years
Gross insured contractual payments outstanding:
Principal	$ 1,469	$ 1,473
Interest	595	600
Total	2,064	2,073
Gross undiscounted claim liability	805	772
Discount, gross claim liability	352	323
Gross claim liability before all subrogation and before reinsurance	(454)	450
Less:
Gross other subrogation	(206)	(208)
Discount, other subrogation	62	60
Discounted other subrogation, before reinsurance	(144)	(149)
Gross claim liability, net of all subrogation and discounts, before reinsurance	(309)	301
Less: Unearned premium reserves	(9)	(10)
Plus: Loss adjustment expenses reserves	1	1
Claim liability reported on Balance Sheet, before reinsurance	301	292
Reinsurance recoverable reported on Balance Sheet	$ 4	$ 3
V [Member]
Schedule of Insured Financial Obligations with Credit Deterioration [Line Items]
Number of policies | Policies	5	5
Remaining weighted-average contract period (in years)	7 years	7 years
Gross insured contractual payments outstanding:
Principal	$ 27	$ 27
Interest	16	17
Total	44	44
Gross undiscounted claim liability	44	44
Discount, gross claim liability	8	8
Gross claim liability before all subrogation and before reinsurance	(35)	36
Less:
Gross other subrogation	(11)	(11)
Discount, other subrogation	3	3
Discounted other subrogation, before reinsurance	(8)	(8)
Gross claim liability, net of all subrogation and discounts, before reinsurance	(27)	28
Less: Unearned premium reserves	(1)	(1)
Plus: Loss adjustment expenses reserves	0	0
Claim liability reported on Balance Sheet, before reinsurance	27	27
Reinsurance recoverable reported on Balance Sheet	$ 0	$ 0